They require water, which requires irrigation (may not already be in place and of course has logistical problems), trees also root systems can lift and crack sidewalks/pipes/other things, you now have biological contaminants, there's bugs and fungus and things that can spread, and so much more.

I'm not saying we shouldn't do this. I really like seeing plant life in cities and I think it really makes a psychological difference (I _love_ living in a city with a lot of green spaces and trees everywhere). But it is nowhere near as simple as placing a tree in the ground and cleaning the leaves.

Tree limbs die all the time. Normally when disease or damage kill a tree branch, it isn't a big deal because the tree is in a forest and probably won't fall on anybody or damage any property under it. In a city, you can't ignore a widowmaker. The risk of it damaging property or flat out killing somebody is too high, you have to pay to get it safely removed. Often this becomes "end-of-life removal" because it's deemed safer or cheaper in the long run to remove the tree entirely. So now you have a green mostly alive tree being chopped down because one of the limbs died, and everybody in the neighborhood thinks you're a tree butcher but nobody else wants to chip in to have it maintained.